Lithium ion secondary batteries are basically comprised of an anode, a cathode, a separator, and an electrolyte, and have been widely applied to small-sized electronic devices, for example, mobile phones, notebook computers, and the like, as high energy density energy storage devices which can be charged and discharged by reversible conversion between chemical energy and electric energy. In recent years, in order to deal with environmental issues, high oil prices, and energy efficiency and storage, application of the lithium ion batteries has been rapidly expanded to hybrid electric vehicles (HEV), plug-in EV, e-bikes, and energy storage systems (ESS).
Although the lithium ion batteries are stable electrochemical devices insulated by separators, short circuit between an anode and a cathode may be caused by internal or external abnormalities of the batteries or by shocks and there is a possibility of heating and explosion. Therefore, ensuring thermal/chemical stability of the separators as insulators is the most important consideration.
Further, in recent years, along with an increasing demand for high capacity and high power output, stability of batteries, that is, stability in explosion and fire, becomes more important. In order to ensure stability of a battery, voltage, current, impedance, temperature, and the like, of a battery cell are generally controlled by an electronic protection circuit or a battery management system (BMS), thereby such attempts responding to abnormalities, such as over charge and over current, of the battery have been made.
A polyolefin-based separator commercially used a lot in lithium secondary batteries is a porous film that prevents short circuit between an anode and a cathode and provides pores serving as a passage of lithium ions. Polyolefin-based separators manufactured by a wet method or a dry method have been widely used commercially.
The wet method is a method including mixing, melting, extruding inorganic particles or oil components with polyolefin in an extruder to prepare a sheet, and forming a thin film by simultaneous or successive biaxial orientation using a roller or a tenter, and extracting the inorganic particles or oil components with a solvent to form a porous film. In this method, polyethylene (PE) among polyolefins is mainly used to form a film. On the other hand, the dry method is a method of forming a porous film by melting and extruding a resin and then orienting the resultant with a roller or a tenter without using an organic solvent, and generally uses polypropylene (PP) and may use polyethylene as necessary.
In a process for preparing a porous separator, except a non-woven separator, a polyolefin porous separator, in which a porous substrate is generally prepared by a film orientation process, cannot avoid a change in volume such as contraction or fusion of its separator when a temperature of a battery is increased to 100° C. or more due to an internal or external stimulus, and, thus, an electrical short circuit between an anode and a cathode may cause explosion. Further, if the separator is broken due to dendrite growth within the battery, an internal short circuit may induce explosion of the battery. There is disclosed a coating separator in which in order to suppress thermal contraction caused by high temperature and instability of the battery caused by dendrite, inorganic particles together with a binder are coated on one or both surfaces of the substrate of the porous separator, and, thus, the inorganic particles suppresses a contraction rate of the substrate and the separator is more stable due to the inorganic coating layer.
In this case, if an organic/inorganic coating layer applied to the porous substrate is not uniformly coated on the porous substrate, when the coating layer is assembled in a secondary battery or within the battery, a part of the inorganic coating layer can be easily separated due to coating defects on the surface. Such separation can decrease stability of the battery. Therefore, a coating system for more uniform organic/inorganic coating is needed to form a uniform inorganic coating layer and ensure an excellent battery property.
As a conventionally well-known technology of an organic/inorganic coating separator, a method for preparing an organic/inorganic coating porous separator by coating organic/inorganic slurry (PVDF-CTFE/BaTiO3 or PVDF-CTFE/Al2O3) with an organic solvent is described in Korean Patent No. 10-0775310. A process of this patent is identical with a conventional electrode solution molding process using a great amount of an N-methylpyrrolidone (NMP) solvent or acetone as a dispersion medium.
Typically, an organic solvent dissolves a binder (PVDF-CTFE) to provide an excellent adhesive property between inorganic particles in powder form when it evaporates. Slurry prepared from a binder solution in an organic solvent provides interconnectivity among a porous substrate, an organic/inorganic coating layer, and inorganic particles within the inorganic coating layer. The components connected as such can endure contraction of a porous separator caused by heating and an external physical shock (event) without losing the interconnectivity during a battery is assembled and operated.
Further, inorganic particles typically has a certain distribution size, and when a binder is completely dissolved in a solvent and the binder has an excellent compatibility with surfaces of the inorganic particles, as the solvent evaporates, the binder sufficiently covers the surfaces of the inorganic particles. Thus, a sufficient adhesive strength between the inorganic particles can be obtained, and even if the binder completely dissolved in the solvent does not have a high wettability with respect to a porous substrate, the binder solution infiltrates into a porous structure and has a sufficient physical adhesive strength with respect to the porous substrate. However, in this case, in order for the binder solution to easily infiltrate into pores of the porous structure and exhibit a relatively sufficient adhesive strength between the inorganic substance and a surface of the porous substrate, the binder in a sufficient amount may be needed, or gel is formed as the solvent volatilizes. Thus, a solvent-impermeable space is generated, resulting in non-uniformity of an organic/inorganic coating layer, which may cause deterioration in battery property. Further, if a concentration of the binder in slurry is increased, a viscosity of the slurry is highly increased, which makes it difficult to prepare an organic/inorganic complex layer of a thin film, and a drying process may require a high temperature. If a low viscosity of the slurry is maintained, an adhesive strength with respect to a porous substrate or between inorganic substances is decreased. Thus, the inorganic particles are easily separated. For this reason, in many cases, a binder dispersed in a certain size in a solvent has been used in the form of an emulsion or suspension, and a binder dispersed in a certain size in an organic (oil-based) solvent may be used, and particularly, a binder dispersed in a certain size in water-based solvent (water) is preferred since it is eco-friendly and has many processing advantages in coating inorganic particles.
However, only with a typically dispersed binder, a sufficient adhesive strength between the inorganic particles or between the inorganic particles and the porous substrate cannot be obtained.
As described above, a coating method based on a binder composition soluble in an organic solvent has some problems. Firstly, a binder soluble in an organic solvent is formed into gel as the organic solvent volatilizes during a drying process, and, thus, a solvent-impermeable space is generated, resulting in non-uniformity of an organic/inorganic coating layer. Thus, a battery property may be reduced. In order to overcome this problem, the binder needs to undergo a secondary drying process in a vacuum at a glass transition temperature (Tg) or higher. If a residual solvent is present in a product due to insufficient drying, a part of the binder is dissolved and gel may be formed. Thus, if a surface of the coating layer becomes sticky, dust from the outside or unnecessary particles may adhere thereto and a defect rate of products may be increased due to adhesion between coating layers or with a substrate when a product is wrapped. Secondly, if a concentration of the binder in the slurry is increased, a viscosity of the slurry is highly increased, which makes it difficult to prepare an organic/inorganic complex layer of a thin film. Further, since air permeability is low and a boiling point is high, a drying process requires a high temperature. Thirdly, if a low viscosity of the slurry is maintained, an adhesive strength with a porous substrate or between inorganic substances is decreased. Thus, the inorganic particles are easily separated. Fourthly, in a process based on an organic solvent, a dry zone of a drying line is elongated due to a critical explosion limit while being dried, and, thus, it is difficult to improve a machining speed. Fifthly, since the organic solvent has volatility, from the moment the slurry is exposed to the external environment, the organic solvent continuously volatilizes. Therefore, a concentration and a rheological property of the slurry are changed due to evaporation of the solvent while the slurry is prepared, transferred, and coated, which may affect a coating quality of a final product. Sixthly, when a coating separator is prepared, risk factors for safety, healthy, and environment are inherent. The organic solvent has toxicity, inflammability, and volatility due to its characteristics. In order to lower the risks of the organic solvent and reduce environmental pollution, the organic solvent needs to be specially prepared and managed. As such, the preparation method of the organic/inorganic coating separator using the binder soluble in the organic solvent has limitations in view of characteristics of the battery and characteristics of the process.
As a method for improving an adhesive strength between a porous substrate and a coating layer, Korean Patent Laid-open Publication No. 10-2012-0052100 describes a technology of preparing a coating separator having two coating layers, comprising: forming an organic/inorganic complex layer by casting a slurry, in which styrene-butadiene rubber (SBR) and carboxyl methyl cellulose (CMC) are dissolved in acetone as an organic solvent, onto a polyethylene porous film; and performing an electric radiation on a polymer compound solution. However, if an organic/inorganic complex layer is formed by this method, the above-described problems of the coating method using the organic solvent also occur. There has been suggested a technology of forming a coating separator in three layers by radiation onto an inorganic coating layer in order to solve the problem caused by separation of inorganic substances due to a low adhesive strength with a substrate. However, in this case, since a film is formed by radiation, it is difficult to overcome the problem of a thickness control of a coating layer at this point in time where a coating separator is demanded to be thinner. Further, due to low uniformity in pore, a current cannot be uniformly distributed but concentrated on a single point when being applied to a battery, and, thus, partial heating, degradation, and explosion may occur. Therefore, it does not provide a fundamental technical suggestion of an organic/inorganic coating separator.
In order to solve a problem that coating materials are easily separated due to a low adhesive strength between a porous substrate and a coating layer when an organic/inorganic coating separator is prepared by the conventional methods, Korean Patent No. 10-1125013 describes a method for preparing a cross-linked ceramic-coated separator using a water-soluble ionic polymer. This method also uses an ionic polymer which can be dissolved in water, but the ionic polymer is not dispersed in water but completely dissolved in water, and, thus, it cannot avoid confinement of the solvent. Since dimethylacetamide as an organic solvent is used 15 times more than water, it does not provide a fundamental suggestion of a coating method using water. In order to induce chemical crosslinking after coating for the purpose of improving an adhesive strength with a substrate, a crosslinking agent and an initiator need to be added together with the organic solvent during a preparation process of slurry, and during a drying process, a heat or UV treatment for 20 hours or more is essentially required. However, if a crosslinking agent and an initiator are added to a slurry solution, before the slurry is applied to the porous substrate, the slurry is partially cross-linked by itself by heat and energy externally applied while a coating solution is stored and transferred, resulting in solidification of the slurry. Thus, uniformity of the coating separator is finally decreased. Further, since a heat treatment and a UV treatment are needed a long time even during a drying process, production yield may be very limited, and a porous substrate of a thin film may be damaged due to high temperature/high energy during the drying process, which may cause deterioration in property and air permeability.